Automatic volume control for telephone lines



March 2o, 192gl `J. HERMAN AUTOMATIC VOLUME CONTROL FOR TELEP HONE LINESFiled Dec. 5l, 1925 1N VEA/TOR JTe/fm/an/ TTORNEY 'Patented Mar. 20,1928.

UNITED STATES PATENT OFFICE.

JOSEPH HERMAN, OP NEW`YORE, N. Y., AssIoNoR 'To AMERICAN TELEPHONE ANDTELEGRAPH COMPANY, A CORPORATION OE NEW YORK.-

A'UTOMATIC VOLUME CONTROL FOB, TELEPHON LINES.v

Application led December 31, 1925i Seri'aIlNo. 78,658.

. This invention relates to telephone transmission systems, and moreparticularly to methods of and means for effecting current volumecontrol in such systems. e l

The invention is particularly applicable to telephone transmissionsystems connecting microphones to distantl radio broadcasting stations.Y

It is the object of applicants inventionto limit automatically thevolume of the telephone currents transmitted over the line withoutchanging the volume at the receiving end.y l The above lstated object isattained in general as follows: At the sending station the peaks of thetelephone currents, when these currents exoeeda predetermined safevalue, are automatically converted into direct current suitable fortransmission over an ordinary telegraph circuit. The direct currentimpulses are caused to effect automatically a reduction of the telephonecurrent volume at the sending station input to the line. Simultaneously,the direct current impulses are transmittedover a telegraph circuit tothe receivingstation, where they automatically eiiect an increase of thecurrent volume at the outputv from the line, thereby compensating forthe volume reduction eftected at the sending station.

Applicants invention will be more clearly lunderstood when the followingdetailed description is read with reference to the accompanying drawing,which shows diagrammatically one desirable form of the circuitarrangements for volume control as applied to a telephone transmissionsystem connecting a microphone to a distant broadcasting station.

With reference to the details of the drawing, the telephone currentsfrom the microphone at the sending station A pass through thepotentiometer P and the amplifier 1 to a gain-controlling circuitincluding the hyy brid coil 2. This gain-,controlling circuit has in ita liXed resistance 25. A variable gain control is obtained through thevariable balance of the hybrid coil 2, which variable balance isIobtained by'varying the plate cir- -cuit impedance of' the vacuum tubeVw, as will be..more fully discussed below. From the hybrid coil 2 thetelephone currents pass through the amplier 3 over the telephone line toa gain-controlling circuit at the receiving station B. This secondgain-controlling circuit includes the hybrid coil 4 and a fixedresistance v26. The variable gain control is obtained through thevariable balance of the. hybrid coil 4, which variable balance in turnis Obtained by varying the plate circuit impedance of the vacuum tubeV3, as. will be more fully discussed below. 'From the hybrid coil 4 thetelephone currents pass through vthe amplifier 5 to the broadcastingstation.

Across the telephone line ,at the points 6 and 7 in the output ofamplifier 3, there is connected through the coil 8 a volume indi.

cater, including the vacuum tube V1.` The grid circuit ot' this vacuumtube includes a battery 27, this battery impressing on the grid anegative voltage ot' such value that normally little or no plate currentows.

` This-vacuum tube permits va direct current to flow in its platecircuit when an alternating wave of suiiicient amplitude is applied tothe grid circuit. Applicants volume indicator differs-from the commontype ymerely in that in place 'ot the usual milliammeter there isconnected in the plate circuit a polar relay R1, which, as will'4 appearmore clearly below, is in effect slow operating and slow releasing. Thispolar relay R1 has .three windings, 9, l0 and 11. 'The winding 9 servesas a biasing winding, and is operated from a local battery through'theresistance 12, in the drawing the plate battery ofthe volume indicatorbeing utilized. for this purpose. Winding 10 is connected into the platecircuit of the vacuum tube V1 and vserves as the main operating windingof the relay. Winding 11 serves as an auxiliary holding winding and isoperated from the positive and negative batteries connected to thecontacts of relay R1, as indicated in the drawing. This windin 11 isconnected from the armature of t e relay 4to ground' in seriesv with theresistance 13 and the capacity 14. It will hold the relay armatureltemporarily on either Contact for a period impulses suitable fortransmission over an ordinary telegraph circuit.

To the armature of relay R1 there are connected the windings of twoadditional polar relays, R2 and R3. As Will appear more clearly below inthe discussion of the operation of applicants system, the relay R2controls the volume reduction at the sending stationby varying the platecircuit impedance of the vacuumtube V2, and relay R3y transmits-thedirect current impulses over the telegraph line to the receiving station"B, where these impulses are caused to eifect a compensating volumeincrease by varying the plate circuit impedance of the vacuum l tube V3.

plate current of the volume in lncrease suliiciently to throw thearmature ofl The -operation of the Volume control system will now bedescribed in detail. For the purpose of illustration, the value of theresistance 25 is chosen as 15,000 ohms, and lthe Value of the impedanceof the tube V2 as from 6,000 ohms to 15,000 ohms. While these values arearbitrarily chosen, they indicatethe difference in values which issuitable. Likewise, at the receiving station B, the value of theresistance 2G is chosen as 5,100 ohms, and the value of the variableimpedance of the tube V3 as 6,000 ohms to 15,000 ohms, as is the casewith the tube V2. When the armatures of. relays R1 and R2 are on their sacing contacts S,- the plate impedance o the tube V2 at one end of thecircuit including the hybrid fcoil 2, differs widely in value from thebalancing resistance 25st the other `end of the circuit. `It is wellunderstood in the art that, given this difference, the hybrid coil 2will offer only-a small loss to the ltelephone currents passing through"it from 'amplifier' 1 to amplifier 3. If the volume levelof thetelephone currents at the inputto the telephone line ein ceeds apredetermined value, the peaks ofthese currents are automaticallyconverted in the volume indicator into pulsating direct current intheplate circuit. This occurs because the voice waves of. amplitude greaterthan the predetermined value .which are impressed uponjthe grid circuitmake the grid potential so much less negative that a substantial amountof current is permitted to flow inthe plate circuit. Conse uently, the

dicator will relay R1 from its spacing contact S to its Vmarking contactM. This operation of the increasing of the amunt of negative voltage onthe grid of the tube Vzincreases the plate circuit impedance of' thattube, as is well understood, and consequently, this plate eircuitimpedance Will be nearer to balancmg4 the 15,000 ohm resistance 25 atthe other en yof the circuit including the hybrid coil 2.-

'line has become low enough to result in the..

release of the relay R1 in the volume indicator circuit. The armature ofrelay Riot course is returned to its spacing contact S, and likewise thearmaturev of relay R2 is returnedv to its contact S. 'The result is thegradual discharge of the condenser through the resistance 19. The levelofv the tele hone currents at the input tofthc. line will) thenriseagain until relay R2 is again operated, and the above describedprocess will be repeated. It will be evident to those skilled in the artthat' the relay operations responsive to the peaks of the voicecurrents, as described above, will. establish .an average voltage on,condenser 18 ot such valuethat the proper input level to the telephoneline will be maintained. 4

As stated above, the operation ofI relay R, results in Vthe operationot' relay Y When the armature of the latter relay is thrown to itsmarking contact M, direct current impulses arevtransmittei'l over thetelegraph circuit to the relay Rl at the receiving station B. This relayacts on the circuits of the -`vacuum tube V3 just as the relay R2 at thesending station A acts on the circuits of the tube V2. When, in responseto'the impulses transmitted over the telegraph line, the armature ofrelay R, is thrown to its markingcontact M, the negative pole of thebattery 20 is connected to the grid circuit of 'the tube V3. Currentfrom the battery 20 flows through the lowpass filter 2l and theresistance 22 and gradually establishes a negative voltage across thecondenser 23. The increase ot' the amount of negative voltage on thegrid of the tube V3 raises'the plate circuit impedance of the tube lfrom6,000 ohms (arbitrarily chosen) toward 15,000 ohms. Since, as statedabove, the value of the lixed resistance 26 in the receiving stationgain-controllingr circuit is 5,100 ohms, `the increase of plate circuitimpedance in the tube Vain response to the operation of relay R4results' in the further unbalancing of the hybrid coil 4.. Consequently,the transmission loss through the hybrid coil is decreased, and thevolume output from the line is increased.

It should be evident from the above deat the input to the line and bycausing a compensating increase of the current volume at the output fromthe line at the receiv-v ing station. Furthermore, it should be evi dentthat this operation is 'rapid and entirely automatic.

In connection With applicants arrange ment for volume cont-rol, therehave been described gain-controlling circuits of a particular type. Itis to be understood that such specification is merely for the purpose ofillustration and that any suitable gaineontrolling means may besubstituted. Also,

4it is to be understood that many other changes from the arrangementsli-own -mai-y be made Without a departure from tlie spirit and scope ofapplicants invention as defined in the appended claims. YVhat is claimedis: l. In a system for the transmission of telephone signals over Wires,the method. of

limiting t'lie current volume in the line,

which consists in automatically converting at the sending station thepeaks'of the telephone currents, when said currents exceed apredetermined value and only at such times, into direct currents, andcausing the direct current impulses automatically to eHect a reductionof the volume at thesending station input to the line andafcoinpensating increase of the volume at the receiving station outputrom'tlie line.

2. In a system for the transmission of telephone signals over Wires, themethod of f limiting the current volume in the line, which consists inautomatically converting atthe sending station. the peaks ofthetelephone currents, when said currents exceed a-.predetermined value andonly at ysuch times, into direct currents, causing the direct cur-y rentimpulses automatically to effect a reduction ofl the volume at thesending'station input to the line, transmitting said impulses to thereceiving stationl over-a separate path, and causing said impulsesautomatically to effect a compensating increase of lthe. volume at thereceiving' station output from the` line.

3. In a system for the transmission of telei `phone signals, said'system including a sending station, a receivingfstation, and. a transhmission line connectingsaid stations, automatic means atthe sendingstat-ion for converting the peaks of the telephone currents,

December, 1925.

when said currents exceed a predetermined phone signals, said systemincluding a sending station, a V receiving station, and a telephone lineconnecting said stations, automatic meansA at the sending station forconverting the peaks of the telephone currents, When said currentsexceed a predetermined value and only at such times, into directcurrents, automatic means at the' sending station controlled bythedirect current impulses for reducing the input volume to the telephoneline, a telegraph circuit for carrying 4. In a system for thetransmission of teletlic direct current impulsesto the receivingstation, and automatic means at the receiving station controlledfby saidimpulsesfor ett'ecting a compensatingmcrease of thev volume of thetelephone currentsat the output from the telephone line.

5. `Ina system for the transmission of teleing station, a receivingstation, and a tele'- phone line connecting said stations, a vacuum tubevolume indicator connected across the input to-the telephone line at thesending sta- `'phone signalsasaid system including a sendtion, aneffectively slow operating and slow releasing relay iii `the platecircuit of said volume indicator, said relay operating when thetelephone currents exceed'a predetermined value, automatic meanscontrolled by said relay for reducing the input volume to 4the`telephone. line, a telegraph lcircuitv be` tween the-"sending stationand th-e receiving station, automatic means controlled by the relay forsending impulses over said tele` graph circuit, and automatic meansat-,the

'receiving station controlled bysaid impulses for effecting lat theoutput from the ltelephone line an increase of the Volume equal to thereduction thereof effected at the sending station.

In. testimony whereof, I have signed` my name to this speciication this30th day of JOSEPH HERMAN.

